The present invention relates to a sublimation thermal transfer recording medium used as an ink ribbon for a sublimation thermal transfer printer and the like. More particularly, the present invention relates to an improvement in resin composition of a thermal transfer dye layer. The present invention also relates to a thermal transfer recording method using the sublimation thermal transfer recording medium.
The thermal transfer recording system using a sublimation dye transfers a large number of color dots to a transfer medium by heating in an extremely short period of time to reproduce a full color image according to an original copy with the color dots of multiple colors.
In the thermal transfer recording system, such a thermal transfer recording medium is used as an ink ribbon that has a base film, such as a polyester film, having formed on one surface thereof a thermal transfer dye layer containing a thermal transfer dye (sublimation dye). The thermal transfer dye layer is superimposed on printing paper, and the back surface of the thermal recording medium is heated according to image information with a thermal head or the like to transfer the sublimation dye contained in the thermal transfer dye layer to the printing paper, whereby a desired dye image is formed. In the case where a full color image is to be formed, thermal transfer dye layers of three colors, yellow, magenta and cyan, which are formed on one surface of the thermal transfer recording medium in parallel to each other, are sequentially superimposed on printing paper to subject to the thermal printing operation. It is also practiced that a thermal transfer dye layer of black color is transferred in addition to those of three colors to form a black image with higher density.
It is important in the thermal transfer recording medium of this type that a printed matter colors in a high density, and the medium causes no failure, such as fusion bonding, with respect to a receiving material, such as printing paper. In this point of view, a vinyl resin, such as polyvinyl chloride, or a cellulose resin has been used as a binder resin of the thermal transfer dye layer of the thermal transfer recording medium.
In order to prevent fusion bonding, it has been also proposed that a silicone graft polymer obtained by modifying an acrylic, polyester, styrene or urethane polymer with silicone, a silicone oil, a phosphate ester and a fluorine surface active agent are added in a small amount to the thermal transfer dye layer as described, for example, in JP-A-9-234963.
In the sublimation thermal transfer recording system, it is demanded that an image with a continuous density from low tone to high tone can be printed, and the thermal transfer dye layer of the thermal transfer recording medium exhibits good correlation between the heat quantity applied and the coloring density, whereby printing with highly accurate gradation is realized.
In the thermal transfer dye layer of the thermal transfer recording medium, a resin having a molecular weight of 100,000 or more and a high glass transition point Tg (about from 70° C. to 90° C.) is generally used as a binder resin to prevent background stain due to after heat of the thermal head from occurring.
In the case where the binder resin has a large molecular weight, however, an ink for forming the thermal transfer dye layer has a high viscosity upon preparation thereof to cause difficulty in production of the thermal transfer recording medium. The binder resin having a large molecular weight is also inert in thermal behavior due to the high glass transition point Tg, and thus it provides a low maximum printing density to cause a problem of shortage in printing density upon applying the medium to high speed printing. In the case where a thermal transfer recording medium having a thermal transfer dye layer using a binder resin having a large molecular weight is used, and an image is directly printed on a surface of a plastic card, such as soft vinyl chloride (containing about 50% of a plasticizer for vinyl chloride), the coloring density is further lowered due to hardness of the plastic card.
In order to solve the problems, it is considered that the glass transition point Tg and the molecular weight of the binder resin used in the thermal transfer dye layer are lowered, but in this case, while the total transferability is improved, another problem arises that background stain occurs on the non-printed area, and high density coloration quickly occurs before the heat quantity is sufficiently increased. Therefore, it is difficult that the background stain is prevented, and printing with accurate gradation owing to good correlation between the heat quantity applied and the coloring density is realized, only by setting the molecular weight and the glass transition point Tg of the binder resin of the thermal transfer dye layer.
On the other hand, it has been attempted that the properties of the thermal transfer dye layer of the sublimation transfer recording medium are improved by adding a silicone material to the thermal transfer dye layer, so as to obtain a sharp printed image. In the case where the silicone material is added to the thermal transfer dye layer, silicone chains are bled out to the surface with the lapse of time to provide an effect of preventing fusion bonding to the receiving material. In the technique disclosed in JP-A-9-234963, a sharp image can be obtained by using a silicone-modified polymer.
However, the silicone-modified polymer used in JP-A-9-234963 is a graft polymer having such a structure that silicone chains are introduced to the main chain (for example, an acrylic chain) in a branched form. Therefore, the silicone chains as side chains are bled out to exhibit the releasing effect, but the main chain remains to stay in the binder to exhibit substantially no barriering effect to the dye. As a result, background stain occurs.
It is considered that the addition of the releasing agent, such as the aforementioned silicone-modified polymer, in a large amount may lower the coloration to suppress the background stain and the like in a certain extent. However, in the case where an ordinary releasing agent or a silicone-modified polymer, such as that disclosed in JP-A-9-234963, is added in such an amount that the state is exhibited, other problems newly arise, such as separation of the dye and repelling thereof upon coating.